Transducer



April 25, 1961 M. DI GIOVANNI TRANSDUCER www April 25, 1961 M. DlGIOVANNI 2,981,912

TRANSDUCER Filed Sept. 30, 1959 2 Sheets-Sheet 2 changes. 1

United States Patent() TRANSDUCER MarioDi Giovanni, Pacific Palisades,Calif.,assignor to Statham Laboratories, Inc., Los Angeles, Calif., acorporation of California Filed Sept. 30, 1959, Ser. No. 843,558 54Claims. (Cl. 338-4) This application is a continuation-in-part of mycopending application Serial No. 762,486, tiled September 22, 1958.

This invention relates to a transducer or load cell capable of measuringforce such as pressure or acceleration, or displacement.

`Conventional load cells have a strain column which is placed undertension or compression by a force summing memben/causing the straincolumn to be either stretched or compressed by the applied force. Theresulting displacement of the strain column corresponding to themagnitude of the force applied is sensed by a motion sensing device. Y

Since the cross-sectional area of the strain column changes duringstretching or compression of the strain column, the deflection of thestrain column against applied load deviates more and more from a linearrelation as the load increases and the cross-section of the straincolumn decreases or increases, depending on whether the strain column isunder tension or compression. The result is that the response becomesmore and more nonlinear with increasing loads, requiring closecalibration of ythe instrument to obtain accurate readings.

An improved form of load cell is described in my above copendingapplication. However, as aforementioned, such load cell, employing anaxially deformable armature, has a nonlinear response for reasons notedabove, namely due to change in the cross-sectional area of the straincolumn when it is deformed by application of a force. Thus, when thestrain column is compressed, the cross sectional area and rigidity ofthe column increases with increase in load and a greater compressiveforce is required to produce a unit of displacement as compared to theforce previously required to produce the same displacement. When thestrain column is placed under tension, the cross sectional area of thecolumn decreases with increase in load, and a reduced tensile force isrequired to produce a unit of displacement ofA the column as compared tothe force previously required to produce the same displacement. Whilethe effects of nonlinearity in the instrument of my above copendingapplication are relatively small, the use of such an instrument isprecludedwhere extremely highA accuracy,y

e.g. 0.10% or less, is demanded, which isL required in many `modernapplications...` e e It is an objectof this invention .to-design atransducer or load 'cell having a substantially lineary response withthe application of force, pressure, or pacceleration.

V yAnother object is to design a load cell having a strainv` column andadeformable armature, said armature being compensatedl in a simple'manner for deviation in det ile'ction of the strain column from alinearrelation with respect 'to theapplied load,4as ther load on the columnOtherobjects and advantages will appear or be obvious from the,following ydescription ofltherinvention. l, ,The vinstant invention isbased ,on the discovery that 1..in-,a load cell comprising ,aldeformable' armature, .a

ice

strain column, the armature being connected at spaced points to thestrain column along its longitudinal axis, wire connections or pinsmounted on the armature in alignment with the axis of said straincolumn, and a strain wire stretched in tension between said wireconnections, if the armature is designed such that it becomeseccentrically loaded bythe applied force to cause secondary bending inthe armature in a direction at an angle, e.g. substantially normal, tothe axis of the strain column, the resulting deflection of the armatureproduces a rocking motion Aof the wire connections or pins, and suchmotion of the wire connections produces a change in tension in thestrain wire which has a compensating effect, resulting in asubstantially linear output from the load cell. Thus, according to oneembodiment, the device comprises a strain column and an armaturepositioned over the strain col-umn, the armature being connected to thestrain column at spaced points axially of the column, the armaturebetween such spaced points being spaced radially of the column to permitthe center portion of the armature to move freely with respect tothecolumn. The armature has an arcuately slotted intermediate portion topermit the armature to readily deform in a direction axially of thecolumn on application of a force thereto. Spaced wire connectionsare'mounted on the armature on a line parallel to the axis of thearmature, and on opposite sides of the slotted portion of the armature.An electrical resistance strain wire is connected to the wireconnections. According to the invention, the armature is designed withbent oiset portions on opposite sides thereof beyond the wireconnections, causing the armature to bend in a direction toward theaxisof the strain column, as well as to deform axially of the column, onapplication of an axial compressive force to the column. Preferably apair of such armatures and associated wire connections and strain wiresare employed, said armatures being positioned diametrically oppositeeach other on the strain column.

Such bending of the armature normal to the strain column (secondarybending of the armature) results in rocking motion of the wireconnections toward each other. Since the strain wire in such embodimentis initially wound near the maximum safe tension and relaxes undercompression of the column, the aforementioned secondary bending of thearmature and resulting motion of the wire connections toward each otheraccelerates relaxation of the strain vwire beyond the normal change inrelaxation thereof due to the compressive force, thus compensating fortheincreasing rigidity of the column under increasing compressive force.The result is a substantially linear `output on change ofapplied load.

Where the instrument is designed as a tensiometer, the bent offsetportions of the armature are designed so that the armature is bent in adirection normal to the axis of the strain column, as well asaxially ofthe c`olumn, on' application of Ytension to the column. The

ingof the armature rocks the .wire conneetionstoward `each other., Thistends to reduce ordecelerate the tension on the wires an amount toycompensate for accel verated elongation of thestrain columnas resultofinf, --creased tensileyload, resulting in a linear response;y Y

The invention willy-be" more clearly understoodwby. reference to; thefollowingjdescriptionof thek invention,

taken in connection with the accompanying drawing Fig. l isalongitudinalzsection through a compression type load cell;

Fig. 2 is/a plan view of one of the armatures mounted on the straincolumn of 1, taken on line of acens-1a Fig. 3 is a section taken on line3-3 of Fig. 1;`

Fig. 4 is a schematic illustration showing the effect on the armaturesin Fig. 1 of the application of a compressive force to the straincolumn;

Fig. 5 is a longitudinal section through Ya modied tension type loadcell; and Fig. 6 is a schematic illustration Vshowing the effect on thearmatures in Fig. 5 of a force applied in tension to the strain columnof Fig. 5.

. Referring to Figs. 1 to 4 of the drawings, the instrument is composedof a strain column 10 which is of elongated cylindrical shape and whichis hollow along its major portion, as indicated lat 10. Over the straincolumn are positioned in diametrically opposite relation a pair of likearmatures 11 which are each mounted on the strain column at the ends ofthe armature. On one end of the strain column is positioned yan integralblock portion on which is mounted a piston 12, the piston having aflange 12 which is fastened, for example, by force'tting or by brazing,about the end block 15 of the strain column. Connected to the outer faceof the piston 12 by suitable means such as brazing is a diaphragm 13,the outer periphery 13 of the diaphragmbeing clamped vbetween the case37 and pressure head 3S. The piston 12 and diaphragm 13 togetherconstitute a force collector or force summing means which transmits anapplied force to the strain column.

Sincereach of the armatures 11 are identical, a descrip- `tion of one ofsuch -armatures will suice, the same numerals being employed torepresent identical corresponding parts on each armature. ri`hearmatures 11 are each essentially in the form of a flat elongated metalstrip. AArcuately shaped flanges 17 and 17' areintegrally connected tothe opposite ends of the armature, and such .-anges are connected as bywelding to the opposite cylindrical shoulders 16 on the outer surface ofthe strain column 10. A pair of offset portions 11a and 11a integrallyconnect the arcuate flanges 17 and 17 respectively of the armature withthe Afiat central portion 11b of the armature. Each of said offsetportions iis bent inwardly `at an angle to the axis of the 'straincolumn 10, as clearly seen in Fig. 1.

The armatures 11 each have a Acentral slotted portion 21 havingy acentral slot 22 which is positioned substantially normal to thelongitudinal axis of the armature. At the opposite ends of slot 22 areformed enlargedapertures23, leaving a narrow metal bridge 24 across thecentrial slotted portion 21 of the armature and connecting the twooppositeY end portions 25fand 26 of armature 11. `ltwill be noted thatmetal strips 24 extend outwardly from the adjacent longitudinal edges 27ofthe armature, and notches 2S are formed in armature portions 25 and26Jadjacent the strips 24. Thus it will be seen that while Aslottedportion 21 ofthe armature 'permitsaxial displacement of one end portion25 of the armature with respect to the other end portion 26, thevprovision of the outwardly extending metal Astrips 24 and the notches28 provide stiffness in the armature adjacent its slotted por-` tion toprevent lateral displacement of the armature portions 25 and 26. Y v

Mounted on 'armature portion 25 adjacent the narrow slot 22 is a pin 30,and disposed on armature portion 26 on the opposite side of slot`22 is apin 31. A strain wire V32' is mounted-in tension on pins 370 and V3-1,the ends of thefstrain wire being connected to terminal pins 33. Anotherpin 34 is mounted on armature-portion 25 of the armature ands'paced'from pinf30. Avv-ire 35V is looped between pin 30 and pin 34, ther endsof the strain wire-being connected-to terminal pins 3.6.Stra`in'wirep32'isan active wirekand wire 3S is inactive;Y and is notinitially mounted in tension.: Pins 30, 31 and 34 are all positionedcentrally of the 4armature on aline parallel to the longitu-` vdinalaxis` of the strain lcolumn 10. Terminal 'pins 33 and 36 are inturnconnected by leads (not shown) to the terminals 33 and 36',respectively, on the armature.;

las.

The Vtransducer is mounted in a case 37 and a pressure head 3S isconnected to one end ofthe case 37 by means of screws 40, the pressurehead 3S and end flange 37 of case 37 clamping the force collector 14 inposition. Pressure head 38 has `an inlet .39 which communicates with thediaphragm 13 of the force collector. In the pressure head is a threadedbleed valve 41 which communicates via passages '41 with the spaceadjacent the diaphragm 13.V

At the opposite end of the case 37 is mounted a cap'42 by means of thescrews 44, there being a ring seal 43 disposed in a recess 43 betweenthe case and the cap 42. The cap 42 carries an inwardly extending flange42 which receives the end 132 of the strain column 10 and holds it inposition by a 'force t or by welding the engaging surfaces. It is thusseen that end 16a of the strain column remains fixed while the oppositeend of 1t) carrying block 15 is displaced relative to end 10a onapplication of a force to the strain column by the force collector 14.

A terminal plate 45 is mounted on the cap 42, and on said terminal plateare a number of terminals 4e to which are connected leads (not shown)which are also attached to the terminals 33 and 36. Mounted on the cap42 by means of the screws 45 is a cover 47 having connected theretoexternal terminals 48.

The strain wires 32 and 35 are connected in a bridge circuit in a wellunderstood manner. The strain wires 32 are the active arms of thebridge, and the strain wires 35' are the inactive arms of the bridge.

A stay plate 49 is mounted in a recess49a of ilange 37 by means ofscrews 49h. The stay plate 49 has a central aperture 49e which receivesthe lange 12 with sufficient clearance to permit axial motion of ange12' butpreventing any undue lateral motion thereof. Oversize holes 49a'are provided for screws 4911 to permit adjustment of the plate.

From the above, it will be seen that when a positive pressure is appliedto the diaphragm 13, the strain column 10 Will be displaced to the rightas viewed in Fig. l. rlhis results in a compression of the strain columnand aiso a compression of the armatures 11 carried by the strain column.This produces a movement of armature portion 25 toward portion 26 and arelaxation and change in resistance of the active strain wires 32 occursproportional to the force compressing the strain column.

However, simultaneously with displacement of the armature 11 to theright, due to the downwardly vbent offset portions 11a and la of thearmature, the armature will become eccentrically loaded as result'of theapplied and reacting forces indicated byfarrows S0 and 51, respectively,in Fig. 4. These forces are applied to the flanges Y17 and 17 at theends of the armature, in a direction parallel to the axis of the straincolumn. The eccentric loading of the armature 11 due to the compressiveforce applied against the strain column 10 causes the armature to bendconcavely inward toward the axis of the strain column, as indicated bythe dotted line 11', producing a rocking of the pins 31 and 34 equallytowards each other, and toward the central pin 30, as indicated indotted lines in Fig. 4. Thus, it will be seen that the relaxation of theactive strain wire 32 is accelerated as the strain `column increases inrigidity as the .load increases. The result is' a compensating 'effectfor the nonlinearity of the ratio of deflection to applied force on thestrain column as the load on thestrain column increases. The measuredoutput fromvthe bridge causedl Vby the applied load is also accordinglymade linear.

y It is thus seen,` viewing Fig. 4, that the bent offset portions 11aand 11a of each of the armatureplatesl 11 aresuiciently resilient to tothe axis of strain column 10, and the armature plates 11 are eachsufiiciently stiffto -undergo secondary bendasindicated bydotted 11,-inresponse to'such bending of-'the ofsetf'portions `11aa`nd 411":1. Theproducbend and to tend-to produce secondary bending of the armature in adirection normaly tion of such secondary bending of each of the armatureplates, simultaneously with axial displacement thereof on application ofan axial force to the strain column, is faciliated by the preferred useof a pair of armature plates, as previously noted on diametricallyopposite sides of the strain column.

It will be noted in Fig. 4 thatsince wires 35 are inactive and notinitially under tension, the rocking of pin 34 toward the central pin30, as result of the secondary bending indicated at 11 of the armature,simply causes further relaxation of wires 3S and does not adverselyaffect the accuracy of response.

It will be understood that substantially all of the applied force beingmeasured is exerted against the strain column to produce compressionthereof, and only an insignificant amount of the applied force isrequired to simultaneously displace the armature in the manner describedabove.

Also, it will be understood that a similar compensating effect toproduce a linear output is realized employing the principles of theinvention for producing a tension type load cell, except that theapplied forces operate in reverse from those described above. Anembodiment of this modification is illustrated in Figs. 5 and 6.

Referring to Figs. 5 and 6, numeral 50 is a strain column having spacedshoulders 52 and 53 thereon. One end of the column has a ange 54 whichis fixed in a rigid block 56. The other end portion 58 of the column isslidably received in an aperture 60 formed in a plate 62, said apertureserving as a guide for longitudinal or .axial displacement of member 5Sand the column 50.

Block 56 and plate 62 are mounted at opposite ends `on a cylindricalcase y64 by means of screws 68, ring seals 66 being clamped between theblock and plate, and

Ythe adjacent case 64. Members 56, 62 and 64 form the frame in which thecolumn 50 is mounted.

A pair of like armatures 70 are positioned diametrically opposite eachother on the strain column. Armatures '70 are each slotted andconstructed substantially the same as armatures 11 described above,except that each of armatures 76 has a pair of spaced apart oisetportions 72 near the opposite ends of the armature, which offsetportions are bent outwardly rat an angle to the axis of the straincolumn instead of inwardly thereto, as in the case of armatures 11. Theoutwardly extending flanges 74 connected to said offset portions 72 atopposite ends of the armature are connected as by welding to theopposite cylindrical shoulders 52 and 53 on tbe strain column 50.

Pins 31' and 34 are mounted on armatures 76 in the same manner and incorresponding location as pins 30, 31 and 34 respectively are mounted onarmatures 11. A strain wire 32 is mounted in tension between pins 30"and 31 of the armatures 70, and a wire 35' is mounted on pins 30 and 34of armatures 7i), strain wires 32 being active and wires 35' beinginactive as in the case of wires 32 and 35, respectively.

When a load is applied in tension in the direction of the arrow in Fig.5, to the rod 78 axially connected to the end portion 58 of the column50, the strain column will be displaced to the left as viewed in Fig. 5.This results in tensioning the strain column and the armatures 70carried by the column, since the opposite end 52 of the column is iixedto the rigid block 56. This produces a stretching and displacement ofthe slotted armatures 70 outwardly; that is, the left portion of each ofarmatures 70 moves awayfrom'the right portion of armatures 70, viewingFig. 5, causing an increase in resistance of the active strain wires 32proportional to the tension vload applied.

Simultaneously` with displacement of the v'armatures Y 70 to the left,the outwardly Ybent oiset portions 72 of.`

the armature cause the armature to become eccentrically ,loaded asresult of the applied and reacting-forces indicated vby arrows Sti inFig. `6. Sucheccentric loading of the armatures 70 due to the tensileforce or load, applied to the strain column 50, causes the armatures'iti to bend concavely inward, as indicated by the dotted lines at 32 inPig. 6, producing a rocking of the pins 31' and 34 towards each otherand toward the central pin 30', also indicated in dotted lines in Fig.6. This causes a relaxation of the tension in the active strain wires32', which relaxation is accelerated as the strain column isincreasingly tensioned and its cross-sectional area increasingly reducedas the load increases. This produces a compensating effect for thenonlinearity of the ratio of deflection to applied force on the straincolumn as the load increases, to thereby cause the strain wires 32 to betensioned linearly with respect to applied load.

As above described with respect to Fig. l, it is noted that therelaxation in tension of the inactive wires 35 has no effect on theoutput, since these wires were not initially wound under tension.

While i have shown the use of two diametrically positioned iilrearmatures, e.g. 11, it will be understood that I may employ a singlecylindrical slotted armature mounted entirely about the strain column,the bent offset portions corresponding to 11a and lla extending entirelyaround the armature.

Also, if desired, the strain column can be in the form of a tube, andthe armature can be mounted within the strain tube and connected atspaced axial points to the tube. l

Although I prefer to apply the invention principles to use of anunbonded strain wire, I can also employ strain wires which are bonded tothe armature, omitting wire connections or pins. In this modification,the strain wires will bend with the secondary bending of the armature toproduce the desired linearity of response. A

While i have described particular embodiments of my invention for thepurpose of illustration, it should be understood that variousmodifications and adaptations thereof may be made within the spirit ofthe invention as set forth in the appended claims. y

I claim:

l. A transducer which comprises a strain member, an armature connectedto said strain member at spaced points Aaxially of said member, saidarmature between said spaced points being spaced radially of said strainmember, said armature having a slotted portion including an arcuate slotlocated between said spaced points, spaced wire connections on saidarmature on opposite sides of the slotted portion of said armature, anelectrical resistance strain wire connected to said wire connections,said armature having offset portions each bent at an angle to the axisof said strain member, said offset portions being located on oppositesides of said slotted armature portion, said oitset portions havingsufiicient resilience to undergo bending on application of a forceaxially of said strain member, and said armature having a stiffness suchthat it bends in a direction normal to the axis of said strain member inresponse to bending of said offset portions, and a force collectorconnected to's'aid strain member, so that on axial displacement of saidstrain member on application of an axial force thereto, Said armaturebends in a direction substantially normal to the axis of said member,producing a rocking of said wire connections with respect to each other,to change the tension in said strain wire and compensate for change incross-sectional area of said strain member with change in applied load.t

2. A transducer which comprises a strain column, an armature connectedto Vsaid strain column at spaced points axially of said column, saidarmature betweensaid spaced points being spaced radially of said column,said armature having an areuately slotted portion intermediate saidspaced points, spaced wire connections on said armature disposedraxiallythereof on opposite sides of theA `slotted portion Tof said armature, anelectrical resistance strain wire connected to said Wire connections,said arma- -ture having bent oiiset portions on opposite sides beyondsaid wire connections, said offset portions having suficient resilienceto undergo bending on application of a force axially of said straincolumn, and said armature having a stiffness such that it bends in adirection normal to the axis of said strain column in response tobending -of said offset portions, so that on axial displacement of saidstrain column on application of an axial force ther,.- to, said armaturebends in a direction substantially normalto the axis of said column,lproducing a rocking of ysaid wire connections with respect to eachother, to change the tension in said strain Wire and compensate -forchange in cross-sectional area of said strain column with change inapplied load.

3. A transducer which comprises a strain column, a pair of likearmatures Vdisposed at diametrically opposite positions on said straincolumn, each of said armatures being connected to said strain column atspaced points .axially of said column, said armatures between saidspaced points being spaced radially of said column, said armatures eachhaving an arcuately slotted portion intermediate said spaced points,spaced wire connections ,on each of said armatures disposed axiallythereof on opl posite sides of the slotted portion of said armatures, an-electrical resistance strain wire connected to said wire rconnectionson each of said armatures, said armatures each having bent offsetportions on opposite sides thereof beyond said wire connections, saidoffset portions having sufl'lcient resilience to undergo bending onapplication -ofa forceraxially ot. said strain column, and each of saidarmatures-.having a stiiness such that it bends in a direction normaltothe axis of said strain column in re- .sponse to bending of said osetportions, so

that on axial displacement of said strain column on application of anaxial force thereto, said armatures each bend in a directionsubstantially normal to the axis of said member, producing a rocking ofsaid Wire connections with respect to each other, to change the tensionin said strain Wires and compensate for change kin cross-sectionalV areaof said column with change in applied force.

4. A transducer which comprises a strain column, an

armature in the forni of an elongated metal plate positioned over saidcolumn, anges integrally connected to -opposite ends-of said armature,said flanges being connected to said strain column at spaced positionsaxially of said column, saidarmature having a siotted central portionincluding an arcuate slot, said armature having a pair of olset portionseach integrally connecting one of saidanges with said central portion ofsaid armature,

veach of said offset portions being bent at an angle to the axis of saidcolumn, said odset portions having sulicient Iresilience to undergobending on application oa force axially ofssaid l,strain column, andsaid Varmature vhaving a stiffness ,such vthat it bends in a directionvnorrnalito theaxis of:said strain column in response to bending of-said olsetportions, pins on said armature disposed on opposite sides ofsaid slot and positioned on a linersubstantially parallel to the axis ofsaid column, anunbonded `electrical,resistance strain Wire connected tosaid pins, .and a ,force collector connected to one end of said column,so that, on axial displacement of said strain column on application ofan axial force thereto, the central portion of said armature betweensaid olset portions bends in a direction at an angle to the axis of saidcoltunn, producing a rocking of said wire connections with respect toeach other, to change the tension in said strain -Wire and compensatefor change in cross-sectional area on, flanges integrally connected toopposite ends of each of. said armatures, said anges being positionedover and connected to said shoulders, said armatures extendinglongitudinally of said column, said armatures. having a slotted centralvportion including an intermediate slot located in a plane substantiallynormal to the axis of said column, said armatures each having a pairofoiiset portions eachsintegrally connecting one of said ilanges withsaid central portion of each of said armatures, each of said offsetportions being bent at an angle to the axis of said column said offsetportions having suicient resilience to undergo .bending 'on applicationof a force axially of saidstrain column, and each of said armatureshaving a stiffness `such that it bends in a direction normal to the axisof said strain column in response to bending of said odset portions,pinson each of said armatures disposed intermediate said'oiset portionson opposite sides of said slot and positioned on a line substantiallyparallel to the .axis of said column, an unbonded electrical resistancestrain Wire connected tosaid pins on each of said armatures, and a forcecollector connected to one end offsaid column, so that, on axialdisplacement of said strain column on application of an axial 'forcethereto, the central portion of said armature between said oiisetportions bends in a direction substantially normal to the axis of saidcolumn, producing a rocking of said pins with respect to each other, tochange the tension in said strain Wire and compensate for change incross-sectional area of said column with change in applied force.

References Cited in the tile of this patent UNITED STATES PATENTS2,920,299 Lent ian. 5, 1960

